1. Field of the Invention
The present invention relates to a method of manufacturing a metal pipe clad cable comprising a metal pipe and a fiber such as an optical fiber housed in the metal pipe, particularly to a method of imparting an excess length to the fiber housed in the metal pipe.
2. Description of the Related Art
An optical fiber is usually in the form of an optical fiber cable consisting of a protective pipe of a metal or non-metal and an optical fiber housed in the protective pipe. When it is used under several conditions as, for example, a marine cable for optical communication, it is required that the optical fiber be protected by a metal pipe having a high corrosion resistance, which is made of, for example, a stainless steel (Japanese Industrial Standards JIS SUS). It is also required that the protective metal pipe be light in weight and small in diameter. An optical fiber cable comprising a protective metal pipe is also adapted for a thermal sensor used in an environment of high temperatures.
FIG. 1 exemplifies an optical communication cable having the configuration described above, showing seven optical fibers housed in an SUS pipe in a bundled state.
If the optical fibers are held stretched within the pipe, fatigue fracture of fibers may occur in any application after a long period of time, which seriously impair the performance of the optical communication cable. It is known to the art that the fatigue fracture occurs in the case where the tensile strain of the optical fiber exceeds 0.2%. Therefore, it is necessary to design and manufacture an optical communication cable in order that the excess length of the optical fiber over the length of the metal pipe is not smaller than -0.2% when installed and also during use of the cable.
FIG. 2 shows a solution to the problem described above, where an optical fiber is loosely inserted into a metal pipe such that the optical fiber has a positive excess length over the metal pipe.
The required excess length naturally differs depending on the applications and environments. In the case of a marine cable previously referred to, the required excess length is substantially 0%. If the cable is used as a thermal sensor for high temperature measurement, however, the expansion of the metal pipe is relatively large composed to that of the optical fiber (substantially 0%). It means that, in some cases, the optical fiber is required to have an excess length of as much as 0.6% in the installation stage. In other words, it is required that the excess length of the optical fiber be accurately controlled as desired to fall within the range of 0% to 0.6% in the manufacturing process of the optical fiber cable.
The two methods given below are known to the art as techniques of manufacturing a metal pipe clad cable:
(1) A long metal pipe prepared in advance is wound around a shaft. While the long metal pipe is vibrated, an optical fiber is inserted into the pipe from one end of the pipe (Published Unexamined Japanese Patent Application No 58-186110).
(2) A long metal tape having a desired thickness and width, which is prepared in advance, is shaped and welded into a pipe. An optical fiber is inserted into the pipe in the welding step (U.S. Pat. No. 4,852,790).
The length of metal pipe clad cable manufactured by method (1) is at most about 2,000 m. On the other hand, method (2) permits the length about 10 times as long as that manufactured by method (1), i.e., about 20,000 m, through the length depending on the length of the metal tape prepared.
When it cause to the control of the excess length, Published Unexamined Japanese Patent Application No. 3-267905 is proposed for method (1), where an elastic strain is imparted in tension to the metal pipe in the step of inserting an optical fiber into a metal pipe in an amount corresponding to the required excess length of the optical fiber, and the tension is released after insertion of the optical fiber into the pipe (hereinafter referred to as method 1-a).
For method (2), the control of the excess length is disclosed in U.S. Pat. No. 4,759,487. It proposes that, in the step of winding up a metal pipe clad cable continuously manufactured, an inert gas is introduced into the manufactured pipe to make the optical fiber pushed, under the control of the pressure, toward the outer circumferential surface of the pipe. In this case, the optical fiber is deviated from the axis of the metal pipe, with the result that an excess length over the metal pipe is imparted to the optical fiber (hereinafter referred to as method 2-a).
The excess length aimed to impart and obtained by method 1-a is only +0.15% as described in Published Unexamined Japanese Patent Application No. 3-267905.
Meanwhile, as described in U.S. Pat. No. 4,759,487, the excess length can be imparted to the optical fiber by method 2-a is +0.2% at the maximum in the case of the particular product specifications and, in a special case, only -0.5%. Unless a desirable excess length is obtained regardless of the product specifications, the practical value of the method is low. In this sense, the scope of applications is limited in the case of method 2-a.
As described above, it is impossible to obtain an excess length of over +0.2% in either of method 1-a and method 2-a. Thus, a large excess length such as +0.6% cannot be imparted to the optical fiber in a metal pipe clad cable. It should also be noted that, in any of the conventional methods, an excess length is imparted in the manufacturing step of a metal pipe clad cable. It follows that, if it is found in the sampling inspection after manufacture of metal pipe clad cables that a desirable excess length is not imparted to the optical fiber, it is impossible to correct the excess length of the optical fiber, as desired. This means that it is unavoidable to discard the undesirable cables, leading to a marked reduction in the product yield. This problem is particularly serious in the case where the excess length control method includes such an unstable factor that the optical fiber is pushed, against the inner wall of the metal pipe, by controlling the gas pressure as in method 2-a. Neither of Published Unexamined Japanese Patent Application No. 3 -267905 nor U.S. Pat. No. 4,759,487 does teach at all how to adjust the excess length of the optical fiber after manufacture of a metal pipe clad cable. In other words, how to solve the problem described above is unknown in this technical field in general.